Method for producing dispersion-strengthened alloys by converting metal to a halide, comminuting, reducing the metal halide to the metal and sintering

ABSTRACT

Grinding mixtures of powdered metals and inert fillers or dispersoids to submicron-sized powders in a gastight mill filled with a hydrogen halide under pressure. The powders are cleaned in hydrogen and compacted.

Tlniied States Patent Arias Mar. 14, 1972 [54] METHOD FOR PRODUCING DISPERSION-STRENGTHENED ALLOYS eferences Cited BY CONVERTING METAL TO A UNITED STATES PATENTS HALIDE, COMMINUTING, REDUCING 3 192 0 2 6/l965 S 1 75/206 pacl THE METAL HALIDE To THE METAL 2,698,990 1/1955 Conant et al. ..75/206 AND SIINTERING [72] Inventor: Alan Arias, Cleveland, Ohio OTHER PUBLICATIONS Goetzel; Tretise on Powder Metallurgy, Vol. 1, Pg. 4; Inter- [73] Asslgnee' The Unwed States America as science Publishers, New York, 1949 represented by the Administrator of the Fazonal Aeronautics and Space Admlnis- Primary Emminer CaflD- Quart-om Assistant ExaminerR. E. Schafer [22] Filed; Nov, 26, 1969 Attorney-N. T. Musial, G. E. Shook and G. T. McCoy [21] Appl. No.: 880,271 [57] ABSTRACT Grinding mixtures of powdered metals and inert fillers or [52] US. Cl. ..75/0.5 BB, 75/213, 75/206 dispersoids to submicromsized powders in a gastight mi filled [5]] 1m. Cl. ..B22f 1/00 with a hydrogen halide under pressure The powders are [58] Field of Search ..75/200, 89.5, 206, 211, 214, cleaned in hydrogen and compacted 75/213, 224, 0.5 BA, 0.5 BB, 0.5 BC; 148/63;

241/16, 18 10 Claim s, 1 Drawing Figure LOAD SEAL EVACUATE MILL MILL MILL PREPARE POWDERS PRESSURIZE MILL WITH HYDROGEN HALIDE I v MONITOR EVACUATE T MILL PRESSURE M'LL POWDERS REPRESSURIZE MILL WITH HYDROGEN HALIDE J I EVACUATE MOVE MILL TO T MILL INERT ATMOSPHERE I REMOVE LOAD POWDERS CLEAN POWDERS INTO RETORT POWDERS HEAT PASS HYDROGEN RETORT THROUGH RETORT ANALYZE RETORT EXHAUST GAS J I PRESSURIZE RETORT MOVE RETORT TO WITH HYDROGEN INERT ATMOSPHERE PREPARE FINAL REMOVE POWDERS V CONSOLIDATE PRODUCT FROM RETORT POWDERS PATENTEIIIIIII I4 I972 3; 649242 LOAD SEAL EVACUATE MILL MILL MILL I PREPARE POWDERS V PRESSURIZE MILL WITH HYDROGEN HALIDE I J I MONITOR EVACUATE PRESSURE MILL I MILL y POWDERS REPRESSURIZE MILL WITH HYDROGEN HALIDE I EVACUATE M MOvE MIL TO MILL INERT ATMOSPHERE Y REMOvE LOAD POWDERS CLEAN POWDERS INTO RETORT POWDERS I I HEAT PASS HYDROGEN RETORT THROUGH RETORT ANALYZE RETO RT EXHAUST GAS J l I PRESSuRIzE RETORT MOvE RETORT TO WITH HYDROGEN INERT ATNlIOSPHERE PREPARE Y I FINAL REMOvE POWDERS CONSOLIDATE PRODUCT FROM RETORT POWDERS J INV-ENTORS ALAN ARIAS ATTORNEYS METHOD FOR PRODUCING DISPERSION- STRENGTHENED ALLOYS BY CONVERTING METAL TO A HALIDE, COMMINUTING, REDUCING THE METAL IIALIDE TO THE METAL AND SINTERING ORIGIN OF THE INVENTION The invention described herein was made by an employee of the United States Government and may be used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention is concerned with obtaining blends of fine metal and dispersoid powders for dispersion-hardened alloys. The invention is particularly directed to milling powders in a reactive atmosphere.

A dispersion-hardened alloy contains well-dispersed, finely divided particles of a material which neither reacts with nor is appreciably soluble in the metallic alloy matrix when heated to the temperature at which the alloy is to be used. Inert materials added to the metallic alloy matrix are usually oxides of high melting point which are more thermodynamically stable than the oxides of the metals that form part of the alloy matrix. The inert substance so used is termed the dispersoid. The amount of dispersoid used depends on the alloy and the properties desired in the final product. This amount usually varies between 0.1 and volume percent.

In most cases, dispersion-hardened metals and alloys have better high-temperature strength than the metals or alloys without dispersoids. One of the main factors governing the strength of these alloys is the interparticle spacing which is the average distance between dispersoid particles. This interparticle space is, in turn, a function of the dispersoid particle size and the volume percentage of the dispersoid.

The interparticle spacing must be maintained for long periods of time at the use temperature if the dispersionstrengthened alloy is to be useful. That is, the dispersoid must be stable. Stable dispersoids have a large negative free energy of formation, high melting point, low solubility in the matrix, and a small disassociation pressure. While there are many potential dispersoids having these characteristics, availability in a suitable size and feasibility of introducing them into the metal or alloy matrix limit the choice. The feasibility of introducing the dispersoids into the matrix, in turn, depends on the method ofdispersing the particles.

Dispersion-hardened alloys are produced by many methods. Most of these methods are applicable only to some specific metal-dispersoid system. The most versatile method of obtaining dispersion-hardened metals and alloys is by grinding or milling. In such a method, the originally coarse metal powders together with the dispersoid are ground or milled to a very fine particle size and simultaneously mixed.

Comminution of the metal powders to the fine particle sizes needed for dispersion strengthening requires the use ofliquids with or without grinding aids as milling media. The milling media contaminates the metal powders with undesirable impurities, such as carbon and oxygen. The powders are then further contaminated during the usual operation of washing, filtering, and drying of the milled powders. In some metaldispersoid systems the contamination may be removed by heating the milling powders in a stream of hydrogen. The oxygen and carbon in reactive metals like chromium, titanium, etc., cannot be removed easily by treating these powders in hydrogen because in some cases no reduction of the impurities is possible. In other cases, the dispersoid agglomerates readily at the temperature required to achieve reduction of the impurities rendering the resulting product practically useless.

SUMMARY OF THE INVENTION According to the invention, relatively coarse metal powders and a dispersoid are simultaneously ground to a very fine powder and thoroughly mixed in a gastight mill filled with one of the hydrogen halides under pressure. The metal powder reacts gradually with the hydrogen halide forming hydrogen and the halide of the metal being milled while the mill pressure decreases. The dispersoid does not react with the hydrogen halide used for milling. The course of the grinding process is monitored with a pressure gage attached to the mill. When the supply of hydrogen halide in the mill is exhausted, the mill is evacuated and refilled with hydrogen halide under pressure. The grinding process is continued until the desired particle size is obtained.

The milled powders are placed in a retort and heated in flowing hydrogen which reacts with the metal halide in the milled powders to form hydrogen halide gas and metal. The hydrogen halide gas is removed by the flowing hydrogen. The exhaust gases from the retort are analyzed for hydrogen halide in order to determine when the metal powders are clean. The cleaned powders are finally removed from the retort and consolidated.

OBJECTS OF THE INVENTION It is, therefore, an object of the present invention to provide improved dispersion-hardened alloys having a minimum amount of undesirable impurities.

Another object of the invention is to provide an improved method of making metal or alloyed powders of fine particle size and with a minimum amount of undesirable impurities.

A further object of the invention is to provide a method of producing dispersion-hardened alloys which involves fewer processing steps than conventional milling methods.

These and other objects of the present invention will become apparent from the specification that follows and from the drawing.

DESCRIPTION OF THE DRAWING The drawing is a flow chart of the preferred method of producing dispersion-hardened alloys.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing a container of a suitable mill is loaded with metal and dispersoid powders. While a ball mill is preferable, a rod mill or vibratory mill can be used. Balls are then added to the powders in the mill container.

The ball mill container is provided with a leaktight cover. The container is likewise equipped with a vacuumtight valve and a pressure gage. After the balls and powders have been loaded into the container the mill is sealed with the leaktight cover.

The mill is then evacuated by connecting the vacuum valve to a suitable vacuum pump. The evacuation is completed before any milling takes place.

The evacuated ball mill container is then filled with one of the hydrogen halides under pressure. The hydrogen halide used should be free from moisture, oxygen, and other undesirable impurities. This kind of hydrogen halide used in one that is capable of reacting with the metals to be milled but not with the dispersoid. In addition, the hydrogen halide is one that produces metal halides capable of being reduced at relatively low temperatures in hydrogen.

The powders are then ball milled by placing the mill in a rack which rotates the container. The pressure in the ball mill decreases during the grinding process because of the reaction of the hydrogen halide with the metal powders being ground. By way of example, in the particular case of chromium being milled with hydrogen chloride gas the reaction is 2Cr(s)+6I-ICl(g) 2CrCl (s)+3I-I (g) As the equation shows, 6 moles of HCI gas react with chromium to form 3 moles of hydrogen gas. Consequently, when all the HCl is consumed in the reaction, the absolute ball mill pressure will be about half the original absolute mill pressure.

The ball milling process is monitored by a pressure gage attached to the ball mill container. This is because the particle size of the metal powder decreases as the amount of hydrogen halide decreases by the reaction with it. Reactions similar to that previously described occur hydrogen halide pairs.

tSrinding of metals to fine particle sizes can also be carried out in liquid hydrogen halides. in this embodiment. however. the grinding process cannot be monitored with a pressure gage because the pressure increases until all the liquid phase is consumed and then it decreases. In addition. the use of liquid hydrogen halides may be dangerous because the increase in pressure due to the release of hydrogen may cause bursting of the mill container.

Rather than use liquid hydrogen halides it is preferable to t'efill the ball mill with hydrogen halide gas as many times as necessary to obtain the desired powder size. When the ball milling is completed as indicated by the pressure the ball milling is stopped. The mill container is removed from the rack and evacuated. The mill container then repressurized with hydrogen halide gas. Ball milling is continued and the pressure is again monitored. These steps are repeated as many times as necessary to obtain the desired powder size.

When the powders reach the desired size the ball ml is again evacuated. The vacuum valve is closed. and the ball mill container is kept under vacuum.

The evacuated ball mill is placed in a glove box containing an inert atmosphere. The ground powder is separated from the halls by sieving in the glove box.

The milled powders are then transferred to the retort within the glove box. The retort has a leaktight cover and is provided with a vacuumtight valve. Other valves are also provided for ponnecting the interior of the retort to hydrogen gas lines.

The retort containing the powders 1S placed in a furnace having suitable temperature controls. The retort is then attached to the hydrogen supply lines. The hydrogen must be of very high purity. such as that obtained by passing hydrogen through a palladium-silver tube hydrogen purifier.

After urging the piping system with hydrogen. the retort is heated to the desired temperature while hydrogen flows over or through the powder in the retort. The metal halide reacts with hydrogen to form solid metal and hydrogen halide gas at a sufficiently high temperature which depends on the metal halide produced during ball milling. In the particular case in which chromium chloride is the metal halide present in the milled powder. the reaction with hydrogen is lECrCl (s)+3H, (g)- 2Cr(s)+6HCl(g) Similar reactions occur between hydrogen and other metal hatides.

The complete reduction of the metal halide is determined by analyzing the gases exhausting from the retort for hydrogen halide. A gas chromatograph is used for this purpose. However. other instruments or processes could be used. By way of example. the exhaust gas could be analyzed by a mass spectrometer. The hydrogen halide could also be collected by cooling the exhaust gas. and then it could be weighed.

After the exhaust gas indicates the powders are completely icleaned. the retort is pressurized with hydrogen. The pressurized retort is transferred to an inert atmosphere glove box. The powders are removed from the retort in this inert atmosphere.

The cleaned powders are then consolidated. This is accomplished in a manner to prevent contamination of the cleaned powders with atmospheric gases. By way ot'example. the powders are pressed into shape inside the glove box and then sintered in a furnace that is also located inside the glove box. Or the furnace may be connected to the glove box through an air lock. It is also contemplated that the powders could be placed tn tubes or cans. Lids are then electron beam welded to the cans which are then isostatically hot-pressed. This latter process requires a glove box that can be evacuated at a very Tow pressure for electron beam welding of the lids. After isobetween other metal.

teaching with a suitable acid.

ttl.

EXAMPLES The invention will be better understood by references to the following examples.

EXAMPLE 1 A stainless steel ball mill was loaded with 150 grams of -30 mesh (5 microns average particle size) chromium powder and 3.800 grams of stainless steel balls. The mill was evacuated and then filled with hydrogen chloride gas at about 150 p.s.i. ibsolute.

The chromium was then ball milled until the mill reached a pressure of about p.s.i.a. The mill was then evacuated, refilled with HCl and ball milling continued. This operation was repeated a total of five times. Total milling time was 425 hours.

The mill was then evacuated and transferred to an argont'illed glove box where the milled powder was separated from the balls by sieving. A small amount of the powder was removed from the box for particle size analysis by the BET method. The as milled powder had an average particle size of 0.043 microns. The bulk of the milled powder in the dry box was placed in a gastight retort. The retort was removed from the glove box and installed in a hydrogen cleaning rig. The rnilled powder in the retort was treated in high-purity flowing hydrogen for 38 hours at 675 C. Analysis of the cleaned powder showed it to have an average particle size of 0.53 microns, and to contain 405 p.p.m. of oxygen, 350 p.p.m. of carbon, 248 p.p.m. of nitrogen, and undetectable amounts of chlorine.

EXAMPLE 2 A stainless steel ball mill was loaded with 141.8 grams of -30 mesh chromium powder. 8.24 grams (representing 4 olume percent) of thoria having an average particle size of 0.01 microns, and 3.800 grams of stainless steel balls. The mill was evacuated and filled with hydrogen chloride under a pressure of about 104 p.s.i.a.

The powders were then milled. The mill was evacuated and then refilled with HCl substantially as in Example 1. Total milling time was 384 hours.

After milling, the mill was evacuated and transferred to an argon glove box where the milled powder was removed from the mill and placed in a retort. A small sample of the power was retained for chemical analysis which showed that the as- ."mlled powders contained 25.4 weight percent chlorine. The retort was transferred to the hydrogen cleaning rig where the milled powders were cleaned in flowing hydrogen for 33 hours it 696 C.

The retort was then pressurized with hydrogen and transferred to the argon glove box where the clean powders were removed from the retort and placed in a stainless steel tube. The stainless steel tube was then sealed under vacuum by electron beam welding. The powder in the welded tube was then tsostatically hot-pressed for 2 hours at 1,093 C. and 10,000 p.s.i.g. Examination of the resulting consolidated chromiumthoria alloy by electron microscopy showed it to have an interparticle spacing (average distance between thoria particles in the photomicrograph) of 3.6 microns.

EXAMPLE 3 Nickel powder weighing 150 grams was ball milled as in the above examples except that hydrogen bromide at 1 l5 p.s.i.a. was used instead of hydrogen chloride. Total milling time was hours. The milled nickel was processed substantially as in Example 1 except that hydrogen cleaning was carried out for 51 hours at 350 C. Chemical analysis showed that the milled powder had 42 weight percent bromine. The particle size of the as-milled powder was 0.023 microns. After cleaning in hydrogen the nickel was still powdery (particle size not determined) and had 0.23 percent of retained bromine, 0.085 percent oxygen. and 0.042 percent carbon.

EXAMPLE 4 The process described in Example 2 was repeated using hydrogen iodide at a maximum pressure of about 70 p.s.i.a. The ball mill was evacuated and then filled with hydrogen iodide a total of five times during the milling run which lasted a total of 384 hours. Chemical analysis showed that the milled powders contained 35 weight percent iodine and the average particle size of the as-milled powder was 0.046 microns. The milled powder was then cleaned in flowing hydrogen for 57 hours at 610 C. The cleaned powder was then consolidated by the same procedure and under the same conditions as in Example 1.

Electron photomicrographs of the resulting chromiumthoria alloy showed an interparticle spacing of 2.1 microns. The thermal stability of this alloy was then determined by heat treating it for I hours at l,3l8 C. in vacuum. After this heat treatment the alloy had an interparticle spacing of 5.2 microns.

EXAMPLE 5 I The process described in Example 2 was repeated using hydrogen bromide instead of hydrogen chloride for ball milling. The milled powders had 46 weight percent bromine and an average particle size of 0.084 'microns. The milled powder was cleaned in hydrogen for 37 hours at 755 C. and then consolidated by the same procedure and under the same conditions as in Example 2. The resulting chromium-thoria alloy had an interparticle spacing of 6.5 microns.

It will be apparent from the above examples that both nickel and chromium can be comminuted to very fine particle sizes by milling them either alone or with a dispersoid in hydrogen halides. The above examples also show that treatment in hydrogen at relatively low temperature reduces practically all the metal halide formed during milling. From this it follows that alloys of chromium and nickel, such as nichrome, with or without a dispersoid can also be milled and subsequently cleaned in hydrogen and consolidated.

EXAMPLE 6 Chromium was ball milled to a very fine powder using solid iodine in an evacuated ball mill that heated to about 100 C. with an infrared lamp during milling. During the milling run the mill was opened several times in the argon glove box and a small sample of the powder removed and treated with normal heptane in which iodine is soluble but chromium iodide is not. ln this manner it was determined that iodine reacts with chromium during milling since after milling for some time heptane no longer showed the characteristic reddish color imparted to it by iodine.

What is claimed is:

1. A method of making a dispersion-hardened metal of the type wherein finely divided particles of a dispersoid are dispersed throughout said metal comprising the steps of comminuting a mixture of powders of said metal and said dispersoid in a hydrogen halide whereby the metal powders react with the hydrogen halide to form metal halide powders,

contacting said metal halide powders with hydrogen to form metal powders and hydrogen halide gas, and

compacting and sintering the comminuted metal powders and dispersoid powders in an inert atmosphere.

2. A method as claimed in claim 1 including heating the comminuted powders in flowing hydrogen to from metal powders and hydrogen halide gas.

3. A method as claimed in claim 1 including comminuting said metal powders and dispersoid powders in a sealed chamber pressurized with a hydrogen halide gas,

monitoring the pressure in the chamber to determine when the hydrogen halide is exhausted,

evacuating the chamber when the hydrogen halide is exhausted, and

repressurizing the chamber with hydrogen halide gas.

4. A method as claimed in claim 3 including the straps of transferring the chamber powders from the scale mill to a retort in an inert atmosphere, and

heating the retort while passing hydrogen therethrough.

5. A method as claimed in claim 4 including analyzing the exhaust gas from the heated retort for hydrogen halide to determine when the metal powders are clean.

6. A method as claimed in claim 1 including the steps of loading coarse metal powders and a dispersoid into a ball mill,

sealing said ball mill,

pressurizing said ball mill with a hydrogen halide gas, and

ball milling said powders in said pressurized sealed mill.

7. A method as claimed in claim 6 including the step of repressurizing said mill with hydrogen halide gas when the original hydrogen halide is exhausted.

8. A method as claimed in claim 7 including the steps of placing the sealed mill in an inert atmosphere after the powders therein are ground to a line powder and thoroughly mixed,

removing said powders from said mill in said inert atmosphere,

placing said powders in a retort,

heating said retort in a furnace, and

passing hydrogen through said heated retort to react with the metal halide powders.

9. A method of making metal powders of fine particle size with a minimum amount ofimpurities comprising the steps of comminuting coarse metal powders in a hydrogen halide gas to form metal halide powders, and

heating said metal halide powders in hydrogen to form metal powders and hydrogen halide gas.

10. A method as claimed in claim 9 including the steps of comminuting the powders in a sealed ball mill pressurized with hydrogen halide gas, and

heating the metal halide powders in a retort while passing hydrogen therethrough. 

2. A method as claimed in claim 1 including heating the comminuted powders in flowing hydrogen to form metal powders and hydrogen halide gas.
 3. A method as claimed in claim 1 including comminuting said metal powders and dispersoid powders in a sealed chamber pressurized with a hydrogen halide gas, monitoring the pressure in the chamber to determine when the hydrogen halide is exhausted, evacuating the chamber when the hydrogen halide is exhausted, and repressurizing the chamber with hydrogen halide gas.
 4. A method as claimed in claim 3 including the steps of transferring the chamber powders from the sealed mill to a retort in an inert atmosphere, and heating the retort while passing hydrogen therethrough.
 5. A method as claimed in claim 4 including analyzing the exhaust gas from the heated retort for hydrogen halide to determine when the metal powders are clean.
 6. A method as claimed in claim 1 including the steps of loading coarse metal powders and a dispersoid into a ball mill, sealing said ball mill, pressurizing said ball mill with a hydrogen halide gas, and ball milling said powders in said pressurized sealed mill.
 7. A method as claimed in claim 6 including the step of repressurizing said mill with hydrogen halide gas when the original hydrogen halide is exhausted.
 8. A method as claimed in claim 7 including the steps of placing the sealed mill in an inert atmosphere after the powders therein are ground to a fine powder and thoroughly mixed, removing said powders from said mill in said inert atmosphere, placing said powders in a retort, heating said retort in a furnace, and passing hydrogen through said heated retort to react with the metal halide powders.
 9. A method of making metal powders of fine particle size with a minimum amount of impurities comprising the steps of comminuting coarse metal powders in a hydrogen halide gas to form metal halide powders, and heating said metal halide powders in hydrogen to form metal powders and hydrogen halide gas.
 10. A method as claimed in claim 9 including the steps of comminuting the powders in a sealed ball mill pressurized with hydrogen halide gas, and heating the metal halide powders in a retort while passing hydrogen therethrough. 